The present invention relates to testing equipment and, more particularly, to test equipment for measuring insulation characteristics of coatings on magnetic materials.
Transformers and motors depend upon flux carrying magnetic circuits for their operation. In a transformer, for example, the magnetic circuit is the core upon which the current carrying windings are wound. The alternating magnetic flux carried by the core transfers power from the input or primary winding to an output or secondary winding. The core or magnetic circuit for carrying alternating magnetic flux is typically built up of thin sheets of magnetic steel with electrical insulation between the sheets. This laminar construction reduces eddy current losses caused by induced electrical currents in the magnetic steel. Eddy current losses result in undesirable power loss or heat in the core material.
Although many types of electrical insulation are known, insulation used in cores of motors and transformers must be capable of withstanding the physical pressures and temperature stresses characteristic of these devices. Furthermore, the insulating material must be relatively thin in order to maximize the magnitude of magnetic steel in the magnetic circuit. In general the insulating material is a coating such as enamel, varnish or an oxide film which is applied to the surface of the steel sheets. The physical characteristics of this interlaminar insulating material and the uniformity of its application are very important since its failure can result in excessive eddy currents with high core losses and possibly localized high temperatures which can result in burning or fusing of the core laminations.
In January of 1947 an article by R. F. Franklin published in the ASTM Bulletin described an apparatus for measuring the electrical insulation characteristics of an insulative coating on magnetic steel. The apparatus has since become popularly known as a Franklin tester. Its description and utilization for testing insulation on magnetic steel is set forth in ASTM Standard A717-75 designated "Standard Method Of Test For Surface Insulation Resistivity Of Single Strip Specimens."
The Franklin tester comprises a contact member which is fastened to a head of a power press and a control box containing a volt-meter, an ammeter, and a regulated power supply. The contact member includes ten probes each terminating in a brass tipped contact, the contacts being forced against the surface to be measured by a spring. A current limiting resistor, usually 5 ohms in value, is connected in series with each contact to limit short-circuit current when the contact is placed against an uninsulated surface. The contacts and resistors are all connected to one terminal of the regulated power supply through the ammeter. The other terminal of the power supply is connected to the magnetic steel sheet being tested so that the resistors and contacts form a plurality of parallel circuits. The voltmeter is connected across the parallel circuits. The series resistances are selected such that for an applied voltage of 1/2 volt, an uninsulated surface will be subjected to a current flow of 1 ampere.
In operation the brass contacts are forced against an insulated steel sheet at a pressure which approximates the pressure within an operating magnetic core. The steel sheet may also be heated to simulate core operating conditions. The power supply is then adjusted until the voltmeter reads 1/2 volt and the current is read from the ammeter. A well insulated sheet will give a low current reading whereas a poorly insulated sheet will produce a reading approaching the 1 ampere calibration level.
During the thirty years in which the Franklin tester has been in use, there have been many instances in which insulation characteristics have been found satisfactory at the manufacturer's facility and unsatisfactory at the core assembly facility. In several instances cross checks of the readings on one Franklin tester with those taken by another Franklin tester have shown substantial disagreement. Tester users have assumed that the variations in readings result from mechanical deterioration, dirty contact tips, point-to-point variations in the characteristics of the insulative coating itself, or from electrical misadjustment during calibration. Mechanical deterioration may appear as a loss of parallelism between the probes thus resulting in a lack of perpendicularity between some of the probes and the steel sheet being tested. Since resistivity measurements depend upon uniform pressure and a constant contact surface area, such deterioration will result in inaccurate measurements.
One attempt to correct for mechanical deterioration has involved polishing the contact tips by placing an abrasive sheet, such as sandpaper, between the contact member and a support while under pressure and then pulling the abrasive sheet from under the contact member. Visual examination of the contact tips indicate a polishing action; however, this process has not resulted in correlated results between testers even when electrical adjustments are carefully and accurately performed.